The present invention relates to a constant volume valve for a combustion-powered tool, such as a power framing tool. More specifically, it relates to a constant volume valve assembly that measures a volume of a fluid before allowing it to flow into a combustion chamber.
This invention also relates to a pneumatically powered, combustion-powered, or other rapidly acting, fastener-driving tool of a type utilizing collated fasteners. Typically, as exemplified in Nikolich U.S. Pat. Re. 32,452, Nikolich U.S. Pat. No. 4,522,162; Nikolich U.S. Pat. No. 4,483,474; Nikolich U.S. Pat. No. 4,403,722 and Wagdy U.S. Pat. No. 4,483,473, which are herein incorporated by reference, a combustion-powered, fastener-driving tool comprises a combustion chamber, which is defined by a cylinder body and by a valve sleeve arranged for opening and closing the combustion chamber. Generally, similar combustion-powered, nail- and staple-driving tools are available commercially from ITW-Paslode (a unit of Illinois Tool Works Inc.) of Vernon Hills, Ill., under its IMPULSE trademark.
In such a tool, it is beneficial to apply a constant force during the driving stroke to each fastener as it is driven into the workpiece. Measurement of the amount of fuel to the combustion-powered tool, or the amount of compressed gas to a pneumatically powered tool, helps provide a constant force. A combustion powered fastening tool is described in U.S. Pat. No. 4,721,240 to Cotta that measures fuel by opening a valve for a length of time defined by movement of a cam. Fuel passes through a fuel valve to a combustion chamber conduit, the amount of which is equal to the volume that passes through a needle valve during the time the fuel valve is open. Measurement of the flow of a fluid by time allows the amount of fluid supplied to the tool to vary as flow rates of the fluid change. As a fuel cylinder is emptied, the flow rate of the fluid changes as the cylinder pressure drops. Similarly, pressure or flow variations in a common supply of pneumatic fluid will also result in differences in the amount of power supplied on each charge of the cylinder.
Control of fuel into a combustion chamber by valve assemblies is shown in U.S. Pat. Nos. 655,996 and 1,293,858. Both references disclose a pressurized fluid inlet valve and fluid outlet valve that bracket a machine-supply passage. High-pressure fluid is fed to a machine to supply power via the inlet valve, and is discharged through the outlet valve when it returns from the machine following expulsion of its power. Neither reference teaches the use of such a system to supply a constant measurement of fluid. Further, following combustion of a fuel or expansion of a high-pressure fluid, the fluid is no longer useful to supply power to a tool and measurement at that point is ineffective.
U.S. Pat. No. 4,913,331 to Utsumi describes an apparatus that drives a piston with an internal combustion engine that utilizes a measuring chamber to dispense a constant volume of fuel. A fuel piston containing the measuring chamber is reciprocally moveable within a fuel cylinder. The fuel inlet channel and the fuel outlet channel are positioned such that the measuring chamber is filled and emptied by movement of the piston between the inlet and outlet channels. Seals are located on either side of the chamber between the fuel piston and the cylinder, preventing leakage of fuel from the pressurized fuel supply to the combustion chamber. Steady movement of the piston would cause rapid wear on these seals, since they are constantly in contact with the cylinder surface.
One operational drawback of conventional combustion-powered tools, is that when operated at relatively low temperatures, such as below 32xc2x0 F., the pressure of the pressurized fuel falls, causing a greater pressure differential between the atmosphere and the fuel. At this lower pressure, the fuel does not dissipate as rapidly through the appropriate passageways and into the combustion chamber. This condition causes a delay in the combustion, which interferes with the operational efficiency of the tool.
It is, therefore, an object of this invention to provide an improved constant volume measurement of a fluid to an apparatus, such as a combustion-powered tool, to produce a constant driving force.
It is yet another object of this invention to provide an improved constant volume measurement of fluid in a compact space.
It is still another object of this invention to provide an improved constant volume valve assembly, whose seals are not constantly wearing against a sealing surface.
It is a further object of the present invention to provide an improved constant volume valve assembly that facilitates the movement of fuel even when fuel pressure drops, such as when the tool is exposed to low temperatures.
These and other objects are met or exceeded by the present device for metering a constant volume of fluid to provide constant energy to a tool. This apparatus is most useful in a portable fastening tool powered either pneumatically or by an internal combustion engine. In the preferred embodiment, configuration of the valves and control mechanism also provides a delay between the closing of one valve and the opening of another, ensuring that fluid is metered before moving downstream to the combustion chamber.
More specifically, the present invention provides a constant volume metering chamber and valve assembly for use with a pressurized fluid supply. The assembly includes housing defining a metering chamber having an inlet and an outlet. A first spring-biased valve is disposed in a housing to control fluid flow to the inlet. A second spring-biased valve is disposed in the housing to control fluid flow to the outlet. An actuator assembly is connected to the two valves, and is sequentially operable from a first position, in which the first spring-biased valve is open and the second spring-biased valve is closed, to a second position, in which the first and second spring-biased valves are both closed, and to a third position, in which the first spring-biased valve is closed and the second spring-biased valve is open. The valve assembly is configured and arranged so that a volume of fluid entering the chamber from the inlet in the first position is collected in the metering chamber, sealed within the metering chamber in the second position, and released from the metering chamber in the third position to provide a constant volume of fluid from each sequential movement of the actuator from the first position to the third position.
The present constant metering valve produces a constant driving force by a fastener-driving tool because it provides a consistent quantity and quality of fuel or hydraulic fluid each time the tool is fired. The fluid supply to the power tool of this invention is measured by volume, not by time, providing a more accurate and more consistent supply of power to the tool. As pressure varies, the fluid density changes in either system because the molecules become more or less densely packed. However, in a flow system, flow rates will also change if the pressure drop across the metering valve fluctuates. Change in flow rate will have no effect in a constant volume system as long as the constant volume chamber is filled in the time the inlet valve to the metering chamber is open.
Further, arrangement of the metering chamber and the spring-biased valves in the present invention leads to compact use of space, as would be useful in a compact, portable tool. Collinear placement of the valves and the oblique angle of the combustion chamber passageway features a shorter distance from the pressurized fluid supply to the combustion chamber, compared to other designs.
Using spring-biased valves to control fluid flow is also advantageous. The seat of the valve that forms the seal with the inlet and outlet of the metering chamber is in contact with the walls of the chamber only for a relatively short time. As the valves open and close, there is no constant rubbing of the seals with adjacent walls. This leads to longer life for the seals.
Another advantage of the present valve assembly is that a disk is preferably provided to at least one of the spring-biased valves which facilitates the flow of fuel into a combustion chamber passageway even in operational conditions when fuel flow is impaired, as when outside operational temperatures fall below freezing.
Still another feature of the present valve assembly is that the actuator assembly is configured to provide an inherent delay in the operation of the upper and lower spring-biased valves to ensure that a designated volume of fuel will be retained in the metering chamber before the lower valve releases the fuel to the combustion chamber. In the preferred embodiment, this delay is achieved in part by a deliberately loose mating engagement between a tongue of an actuator pivoting link arm and a notch in an actuator control arm. This loose engagement ensures that, while the pivoting link arm travels a continuous motion due to the engagement of the tool upon a workpiece, the actuator control arm is not continuously moved, resulting in a slight xe2x80x9cpausexe2x80x9d in the operation of the spring-biased valves. In this manner, the consistency of the volume of fuel temporarily held in the metering chamber is maintained.